DESCRIPTION: Phase change in plants is the process of predictable changes that occur during vegetative growth. These changes are not always as dramatic as the changes that occur in the transition from vegetative to reproductive development. These changes may include changes in leaf shape, in the epidermis, or in the presence of trichomes. In other work from Dr. Poethig's laboratory, dominant mutants in maize designated as Teopod1, Tp2, and Tp3 have been identified. These mutants prolong the presence of juvenile traits. A second mutation, glossy 15, results in the epidermis of juvenile plants assuming an adult fate. Because these events do change the time that plants begin to respond to environment stimuli, it is possible that during development there may are a set of clocks allowing timing of different types of events. Mutations that affect phase change fall into two classes. They affect the duration of the early phase and could be candidates for resulting phase changes or they may be mutations that affect the duration of the late phase and act to regulate floral initiation. Dr. Poethig and the postdoctoral Dr. Telfer have screened for mutations in Arabidopsis that change the timing of the transition from juvenile to adult during vegetative growth. The markers for juvenile and adult phases are the presence of trichromes and the shape of the leafs. Four stages can be recognized. Early rosette leaves have trichomes on the upper leaf surface, late rosette leaves have them on both surfaces. Early bract or cauline leaves have them on both surfaces and late bract leaves have them only on the lower surface. The screen was for plants that changed the timing of the appearance of trichomes on the lower surface of rosette leaves and/or changed the timing of the appearance of trichromes on the upper surface of bract leaves. 2,000 M2 mutagenized plants were screened. Two mutations that accelerate the appearance of late phase markers were isolated. The two loci are psd (paused) and hst-1 (hasty). Plants homozygous for psd germinate and then pause for several days before starting development. This mutant may suggest that the identity of the leaf is determined by a temporal mechanism rather than a spatial one. The second locus, hst-1, has also been identified and it appears to accelerate all phases of vegetative growth. Dr. Poethig intends to continue the screen for additional mutations that accelerate the juvenile phase. Mutations that prolong the early phase are apparently more difficult to recognize. Given the result observed in hst-1 with changes in flowering, existing mutations that affect the timing of flowering will also be examined for changes in the timing of phase changes. Dr. Poethig will examine double mutants with the goal of distinguishing between two different models for the regulation of phase changes. They involve one pathway or two pathways. The one pathway model suggests that a single pathway controls both vegetative and reproductive development. The second model suggests that one pathway controls vegetative changes and a second controls reproductive changes. Grafting experiments have suggested the presence of a juvenile factor that is produced by roots or young leaves. The juvenile factor would suppress flowering and rejuvenate the vegetative parts of the plant. The hst-1 mutation has the phenotype that might be expected of a gene for the factor or a gene that responds to the factor. The site of hst action and its cell autonomy will be investigated by reciprocal micrografting experiments using wildtype and mutant roots and shoots. The final goal is to clone the HST-1 gene by positional cloning from a linked molecular marker in YAC clones. The physical and genetics maps will be aligned using recombinants between hst-1 and a molecular marker to one side. Once the region containing the gene is spanned, the assay will be complementation and allele-specific polymorphisms.